The invention relates to a method and means for transmitting data through a drill string in a borehole.
While drilling boreholes in underground formations, information on a number of downhole parameters, such as azimuth, inclination, tool face, weight on bit, borehole temperature, pressure, and downhole motor speed, should be available to the drilling crew to guide them in making the correct decisions during progress of the drilling operation.
To this end, sensors are placed near the drill bit, which sensors are suitable for obtaining data on specific parameters. The data are subsequently encoded in a form suitable for transmission thereof to the surface, where they are decoded and transformed to a form suitable for interpretation by the drilling crew.
The present invention relates in particular to a method and means for transmitting data to the surface in the form of pressure variations through the drilling liquid or mud in the drill string. Such pressure variations (also indicated as acoustic waves) may be in the form of pressure pulses, and are generated in the column of drilling mud present in the drill string by varying the resistance that the flow of drilling mud meets in the lower parts of the drill string. In the systems applying acoustic waves for data transfer, the measured values of a particular parameter are encoded in a pattern of electric signals that control the position of a valve in the drill string in such a manner that the valve is successively opened and closed stepwise according to the pattern, thereby raising pressure variations in accordance with this pattern in the column of drilling mud. The pressure variations travels upwards through the drilling mud to the surface, where they are detected by a pressure sensor lcoated in the supply conduit of the drilling mud to the drill string. The pattern of pressure variations that represents a particular value of the measured parameter is then decoded and the value of the parameter is presented to the drilling crew in a form suitable for a ready understanding, and/or stored in a suitable form for later study.
The presently most common way of opening and closing the valve for generating the pressure variations, is by actuating the valve by means of a solenoid. The valves, however, rapidly wear away by the erosive action of the drilling mud that passes the valves in turbulent flow. Moreover, the actuation of the valves consumes a considerable amount of electric energy, which energy should be made available from batteries located in the drill string and/or be generated in-situ by electric generators driven by liquid turbines that are located in the mud flow passing through the drill string. The use of large-sized batteries is not attractive since they are rather costly and require a large space in the drill string. The use of hydraulically actuated turbines is not attractive in view of the high costs of the turbines and their liability to plugging and wear by the drilling mud.